arm: Revert Auto-vectorization for MVE: add pack/unpack patterns PR target/104882

arm: Revert Auto-vectorization for MVE: add pack/unpack patterns PR target/104882

This reverts commit r12-1434-g046a3beb1673bf to fix PR target/104882.

As discussed in the PR, it turns out that the MVE ISA has no natural
mapping with GCC's vec_pack_trunc / vec_unpack standard patterns, unlike
Neon or SVE for instance.

This patch also adds the executable testcase provided in the PR.
This test passes at -O3 because the generated code does not need
to use the pack/unpack patterns, hence the use of -O2 which now
triggers vectorization since a few months ago.

2022-03-18  Christophe Lyon  <christohe.lyon@arm.com>

	PR target/104882
	Revert
	2021-06-11  Christophe Lyon  <christophe.lyon@linaro.org>

	gcc/
	* config/arm/mve.md (mve_vec_unpack<US>_lo_<mode>): Delete.
	(mve_vec_unpack<US>_hi_<mode>): Delete.
	(@mve_vec_pack_trunc_lo_<mode>): Delete.
	(mve_vmovntq_<supf><mode>): Remove '@' prefix.
	* config/arm/neon.md (vec_unpack<US>_hi_<mode>): Move back
	from vec-common.md.
	(vec_unpack<US>_lo_<mode>): Likewise.
	(vec_pack_trunc_<mode>): Rename from
	neon_quad_vec_pack_trunc_<mode>.
	* config/arm/vec-common.md (vec_unpack<US>_hi_<mode>): Delete.
	(vec_unpack<US>_lo_<mode>): Delete.
	(vec_pack_trunc_<mode>): Delete.

	PR target/104882
	gcc/testsuite/
	* gcc.target/arm/simd/mve-vclz.c: Update expected results.
	* gcc.target/arm/simd/mve-vshl.c: Likewise.
	* gcc.target/arm/simd/mve-vec-pack.c: Delete.
	* gcc.target/arm/simd/mve-vec-unpack.c: Delete.
	* gcc.target/arm/simd/pr104882.c: New test.